ovs/gre: fix rtnl notifications on iface deletion

The function gretap_fb_dev_create() (only used by ovs) never calls
rtnl_configure_link(). The consequence is that dev->rtnl_link_state is
never set to RTNL_LINK_INITIALIZED.
During the deletion phase, the function rollback_registered_many() sends
a RTM_DELLINK only if dev->rtnl_link_state is set to RTNL_LINK_INITIALIZED.

Fixes: b2acd1dc ("openvswitch: Use regular GRE net_device instead of vport")
CC: Thomas Graf <tgraf@suug.ch>
CC: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>